1. Field of the Invention
The present invention relates to an optical encoding method and optical encoder for use in optical communication. More particularly, the present invention relates to optical encoding using a time-spreading wavelength-hopping code.
2. Description of the Related Art
Optical multiplexing raises the capacity of optical communication systems by enabling a single transmission path to carry a plurality of communication channels. Various optical multiplexing methods have been developed, starting with time division multiplexing and proceeding to wavelength division multiplexing, which provides more communication capacity. Even higher communication capacities are expected to be achievable by optical code division multiplexing (OCDM), which permits a plurality of communication channels to share the same time slot and the same wavelength group.
A nine-chip, three-wavelength optical encoder using a time-spreading wavelength-hopping code for OCDM and employing chirped fiber Bragg gratings (CFBGs) is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2000-209186.
Encoders of this type were used in a transmission experiment described in a paper by Naoya Wada, Hideyuki Sotobayashi, and Ken-ichi Kitayama entitled “Time-spread/wavelength-hop OCDM using fiber Bragg grating with supercontinuum light source”, 1999 IEICE Communication Society Conference, B-10-128. The data transmission rate in this experiment was 2.5 gigabits per second (Gbps). This rate and the disclosed dispersion characteristics indicate that the optical encoders were forty millimeters (40 mm) long and used CFBGs substantially 9 mm in length.
In the encoding process described in these documents, a wideband light pulse 1 (FIG. 10A) including wavelengths λ1, λ2, and λ3 (FIG. 10B) is input to an optical encoder 2 (FIG. 10C). The optical encoder 2 is an optical fiber of length L having three internal CFBGs with grating pitches Λ1, Λ2, Λ3 respectively disposed at positions L1, L2, L3 on the longitudinal fiber axis or z-axis. Three reflected optical pulses with different wavelengths λ1, λ2, λ3 and different delays are output from the optical encoder 2 for transmission to a distant decoder. Viewed on the time axis (t), a single input pulse 1 (FIG. 10A) has been converted to an optical pulse train 3 (FIG. 10D) including separate pulses with wavelengths λ1, λ2, λ3. The nine chips refer to nine positions on the time axis at which the three pulses with wavelengths λ1, λ2, λ3 may occur.
In the decoding process, the optical pulse train 3 (FIG. 10E) including wavelengths λ1, λ2, λ3 (FIG. 10F) is directed into an optical decoder 4 (FIG. 10G) having CFBGs positioned in a mirror-image relationship to the positions of the CFBGs in the optical encoder 2. The pulses are thus reflected with delays that compensate for the delays produced in the optical encoder 2, so that the optical pulse train 3 is restored to a single optical pulse 5 (FIG. 10H) in which the λ1, λ2, and λ3 wavelength components have the same timing.
The optical encoder 2 functions as a high-precision time-spreading, wavelength-hopping control element. Incidentally, wavelength hopping is also referred to as frequency hopping, and optical code division multiplexing is also referred to as optical code division multiple access (OCDMA).
The optical encoder 2 used in the prior art described above produces relative delays shorter than the input pulse period, so that the encoded pulse trains do not overlap. While this non-overlapping condition prevents interference, it also limits the data transmission rate. An optical encoder 2 with a total length of 40 mm, for example, is limited to a maximum data rate of 2.5 Gbps.
The number of codes available for multiplexing is also limited. One reason is the limited number of chips into which each input pulse period can be divided, since the chip interval cannot be shorter than the input pulse width. Another reason is that to obtain the necessary spectral shape, the CFBGs used in the optical encoder must have lengths from substantially 2 mm to 10 mm. Since adjacent CFBGs must be physically separate, if the chip interval is reduced to provide more chips, the pulses in the encoded pulse train must be separated by an increasing numbers of chips. This requirement constrains the pulse-train structure so that the shortened chip interval fails to produce a matching increase in the number of codes. A further restriction is that when different optical signals are multiplexed by the use of different optical codes, to avoid inter-code interference, no two codes may have the same wavelength element located at the same chip position.
If each encoder produces only delays shorter than the input pulse period, there is accordingly a tradeoff between the data transmission rate and the number of channels that can be multiplexed. As the data transmission rate increases and the input pulse period is reduced, the maximum code length (number of chips) is reduced, and the number of codes available for multiplexing becomes highly restricted. In the examples of the prior art described above, in which three wavelength elements are spread over nine chips on the time axis but cannot occupy adjacent chip positions, a maximum of six channels can be multiplexed.